Pixel having an organic light emitting diode and method of fabricating the pixel

ABSTRACT

A pixel having an organic light emitting diode (OLED) and method for fabricating the pixel is provided. A planarization dielectric layer is provided between a thin-film transistor (TFT) based backplane and OLED layers. A through via between the TFT backplane and the OLED layers forms a sidewall angle of less than 90 degrees to the TFT backplane. The via area and edges of an OLED bottom electrode pattern may be covered with a dielectric cap.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application ser. No.13/112,654, filed May 20, 2011, which is a continuation of U.S.application Ser. No. 10/546,695, filed May 18, 2006, which is a U.S.National Stage of International Application No. PCT/CA2004/00256, filedFeb. 24, 2003, which claims priority to Canadian Patent No. 2,419,704,filed Feb. 24, 2003, each of which is hereby incorporated by referenceherein in its entirety.

FIELD OF THE INVENTION

This invention relates to a pixel, more particularly, to a pixel havingan organic emitting diode.

BACKGROUND OF THE INVENTION

Organic light emitting diodes (OLEDS) are electro-luminescent (EL)devices for emitting light. The OLED generates light by a currentflowing through an organic compound. Pixels including the OLEDs havevarious advantages, i.e. simple structure, fast response and wideviewing angle. There are two types of matrix displays with the OLEDs,passive type and active type. In the active matrix display, thin-filmtransistors (TFT) are provided in each pixel to drive the OLEDs ofdisplay. The active matrix eliminates high peak driving currents andthereby enables high-resolutions and high information density, improvespower consumption and life-time compared to the passive matrix.

Vertical pixel architecture, in which the TFT and the OLED device arestacked vertically, has been developed. Such architecture can achievehigher aperture ratios. This favors using lower mobility amorphoussilicon TFT backplanes compared polysilicon TFT technology, which is ofhigher mobility but also of higher cost.

The difficult part in building the vertical stacked pixels is to make aTFT backplane suitable for subsequent OLED fabrication and provide highyield and good performance of OLED pixels. The OLED device is typicallymade of very thin layers. Overall thickness of organic layers in theOLED is of the order of 100 nm. For this reason, it requires a smoothsubstrate to achieve good performance and yield. Step-wise features onthe substrate surface and roughness can cause deterioration oflight-emitting properties or OLED device failure due to shorts betweenits electrodes.

It is, therefore, desirable to provide new pixel architecture, which canachieve a high aperture rate, and at the same time, higher yield rate.

SUMMARY OF THE INVENTION

It is an object of the invention to provide novel pixel architecturethat obviates or mitigates at least one of the disadvantages of existingpixels.

In accordance with an aspect of the present invention, there is providedvertical pixel architecture in which a planarization dielectric layer isdisposed between a TFT based backplane and OLED layers. Theplanarization dielectric layer is thick enough to smoothen a TFTsubstrate profile to such an extent that will make it suitable forsubsequent fabrication of the OLEDs. Preferably, the planarizationdielectric and subsequent electrode layer have a roughness of the orderof 1 nm to permit successful OLED fabrication.

Electrical connection between TFT circuit and OLED is provided by meansthrough-via made in planarization dielectric.

In accordance with a further aspect of the present invention, there isprovided a vertical pixel architecture in which continuous sidewallcoverage is provided by pixel electrode material in a through-viaprofile provided in the planarization dielectric. This is achieved bythe formation of sloped sidewalls of the through-via. Preferably, theangle between the via and a TFT substrate is less than 45 degrees.

In accordance with a further aspect of the present invention, theinterconnection between TFT final metal and OLED bottom electrode invertical pixel architecture is provided via a smooth contact plate madeof conductive material.

In accordance with a further aspect of the present invention, there isprovided a vertical pixel architecture in which a dielectric layer isdeposited and patterned on the top of the pixel electrode in such a waythat it covers pixel via and the edges of the pixel electrode.

Other aspects and features of the present invention will be readilyapparent to those skilled in the art from a review of the followingdetailed description of preferred embodiments in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further understood from the following descriptionwith reference to the drawings in which:

FIG. 1 is a schematic cross-section view showing a vertically integratedpixel in accordance with an embodiment of the present invention.

FIG. 2 is a schematic cross-section view showing an example of the pixelof FIG. 1.

FIG. 3 is a schematic cross-section view showing an example of the pixelof FIG. 1, which incorporates a shield electrode.

FIG. 4 is a schematic diagram showing an example of surfaceplanarization with BCB.

FIG. 5 is a schematic cross-section view showing a sidewall slope .beta.of the pixel of FIGS. 2 to 3.

FIGS. 6 to 8 are schematic diagrams showing fabricating process of thepixel of FIG. 2.

FIG. 9 is a schematic cross-section view showing an example of the pixelof FIG. 1, which incorporates a contact plate.

FIG. 10 is a schematic cross-section view showing an example of thepixel of FIG. 1, which incorporates a shield electrode and a contactplate.

FIG. 11 is a schematic cross-section view showing a verticallyintegrated pixel in accordance with another embodiment of the presentinvention.

FIG. 12 is a schematic diagram showing fabricating process of the pixelof FIG. 11.

FIG. 13 is a schematic diagram of a vertically integrated pixel inaccordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A vertically integrated pixel of the present invention is described.FIG. 1 shows a vertically integrated pixel 10 in accordance with anembodiment of the present invention. The pixel 10 includes OLED devicelayer 12 and a TFT based backplane 14 (hereinafter referred to as TFTbackplane).

The OLED device 12 includes one or more organic layers, a cathode and ananode. In the description, layers between the cathode and the anode arereferred to as OLED layers 18. The OLED layers 18 may be incorporatingan electron transport layer, an organic light emitting layer, a holetransport layer, and a hole injection layer. In FIG. 1, an OLED topelectrode 16 and an OLED bottom electrode 20 are shown as the cathodeand the anode, respectively.

The top electrode 16 is transparent to enable the light to be emitted bythe OLED in the direction opposite to the substrate (i.e., top-emittingOLED). However, reverse top-emitting OLED structure, where the bottomelectrode 20 is a cathode, and the top electrode 16 is an (transparent)anode, is also possible.

Each pixel of the TFT backplane 14 includes TFT pixel circuits formed ona substrate 30. In FIG. 1, two TFTs T1 and T2 form a pixel circuit. Eachof the transistors T1-T2 has metallization for a source, a drain and agate 6. In FIG. 1, “2” represents either a source node or a drain node.However, the pixel 10 may include more than two transistors.

The OLED bottom electrode 20 is formed on the top of the TFT backplane14, and is separated from the backplane 14 by a dielectric layer 22. Thedielectric layer 22 is continuously provided everywhere on the top ofthe TFT pixel circuit except at a through-via 8, which provideselectrical connection between a specific node of the TFT pixel circuitand the OLED bottom electrode 20. This specific node may be source nodeor drain node of a TFT, which depends on pixel circuit design and orderof deposition for the OLED electrodes and layers. The details of circuitdesign and OLED fabrication are not to restrict the applicability of thepresent invention.

Preferably, the planarization dielectric and subsequent electrode layerhave a roughness of the order of 1 nm to permit successful OLEDfabrication. Optionally, a shield electrode 24 is provided on the top ofTFTs.

FIG. 2 shows an example of the pixel 10 of FIG. 1. In FIG. 2, thesidewalls of the through-via 8 are sloped. The OLED bottom electrodematerial is disposed on the top surface of the dielectric layer 22 andalong the sidewall of the sloped through-via 8.

FIG. 3 is another example of the pixel of FIG. 1. In FIG. 3, the shieldelectrode 24 is provided above the TFT layers to keep the potentialright on the top of the TFT pixel circuit at certain designed levelregardless of the potential of the pixel electrode. The shield electrode24 may be a thin-film conductor, Al, Al-alloy, Mo, Cr or the like. Aninterlayer dielectric 21 is provided between source/drain and shieldlayers. The connection between the desired pixel circuit node and theOLED bottom electrode 20 is made by means-of a via in the interlayerinsulator 21, an interconnection plate 26 formed in the shield metallayer and the through-via 8 formed in the dielectric layer 22.

The transistor structure of FIGS. 1-3 is typical for bottom-gateamorphous silicon TFT, and it is shown here as one possible exampleonly. However, the method of pixel integration described here may beapplicable in general to any appropriate known TFT backplane, includingrecrystallized or deposited poly-silicon, micro- and nano-crystallinesilicon, CdSe and others.

Active matrix TFT backplane may be fabricated by successive depositionand patterning of metal, insulator and semiconductor layers leading toan overall profile height of the structure that is in the range of a few100 nm to 1 micron, with nearly vertical or sharp-angled sidewalls ofthe structures. On the other hand, in high performance small moleculeand polymer organic light emitting devices, active organic layers havean overall thickness in the range of 10-100 nm. This implies that it isdesirable to provide the OLED substrate with the roughness in 1 nm rangeto prevent electrical shorts between OLED layers or top and bottomelectrodes. In addition, it is desirable that the substrate is eitherplaner or has sufficiently smooth features whose vertical profile doesnot prevent reliable step coverage with thin OLED layers and theircontinuity, where necessary.

In the embodiment of the preset invention, the planarization dielectricand subsequent electrode layer are formed so as to have a roughness ofthe order of 1 nm. The dielectric layer 22 smoothens or planarizes thevertical profiles of the structures on the substrate with fabricated TFT14. Further, the through-via profile in the dielectric layer 22 enablescontinuous sidewall coverage by the OLED bottom electrode material, andreduction of thickness of the pixel electrode.

The dielectric layer 22 of the pixel 10 is described in detail. Thedielectric layer 22, which is used for separating the TFT backplane 14and the OLED bottom electrode 20, smoothens or planarizes the verticalprofiles of the structures on the substrate 30 with the fabricated TFTbackplane 14. This ensures continuity of the electrodes 16, 20 andorganic layers 18 in the OLED device 12. This smooting/planarizing isachieved by using a planarizing dielectric, rather than one coating thesubstrate conformally. The planarizing dielectric may be an organicpolymer such as benzocyclobutene (BCB), polyimide, polyamide, acrilicand others. Minimum thickness of planarization layer required depends onplanarization properties of the dielectric and the profile height of TFTbackplane. The thickness of planarizing dielectric can be between 0.5and 5 .mu.m. In the embodiment of the present invention, BCB layer,about 3 micron-thick, produced from photosensitive BCB-material is usedas the planarizing layer.

Planarizing dielectric layers are most often produced by application ofcorresponding initial material or monomer, which can be polymerized onthe substrate by means of thermal cure, UV-cure with our withoutcatalyst or by other method. The initial or monomer material can orcannot be patterned by photo-exposure. This property depends on chemicalformulation of initial material or monomer by the manufacturer, whetherthe photosensitive components were added or not. The processing of theformer may include steps such as application of initial material,pattern definition by photoexposure trough a photomask, patterndeveloping and final cure. As a result a patterned polymer layer isobtained. The processing of the latter may include application ofinitial material, cure, application and patterning of the mask,patterning cured polymer by means of plasma or wet etching with themask, strip the mask. In some cases, like polyimides and BCB, there areavailable both photosensitive and non-photosensitive versions of initialmaterial that can lead to about the same chemical composition andstructure of polymer dielectric material after final cure.

In the embodiment of the present invention, BCB-layer made ofphotosensitive initial material is used as a planarization dielectric.However, the present invention may be applicable to different types ofmaterial, such as but not limited to other planarization materials madeof both of photosensitive and non-photosensitive initial formulations.

FIG. 4 shows the planarization effect of BCB-layer. In this example, aTFT substrate is schematically shown as-having stepwise profile of thepatterns 50 with nearly vertical sidewalls and profile height of 0.5-0.9.mu.m before application of BCB. After application of BCB-film, thepatterns 50 are translated into 0.3-0.5 micron profiles 52 with thesidewall angle a about 10 degrees on the surface of BCB dielectric. Inthis example, the BCB-polymer film was produced by spin-coatingphotosensitive material (photosensitive BCB) with subsequent soft bake,exposure, post-exposure bake, pattern developing, solvent removal andcure. The process conditions are shown in Table 1.

TABLE 1 Process conditions for planarization layer Nr Step Conditions 1Spin-coating 2500-4000 rpm, 25-40 sec 2 Soft-bake 60-70 degree C., 90sec 3 Exposure 12-60 sec 4 Post-exposure bake 50-60.degree C., 30 sec 5Developing 2-4 min 6 Solvent removal 75 degree C., 60 sec 7 Cure190-250.degree C., 2-4 hrs

The through-via profile in the dielectric 22 of the pixel 10 is nowdescribed in detail. The OLED bottom electrode 20 is a conductivematerial such as indium-tin oxide (ITO) or the like, a metal film, Au,Pd, Ni or the like, sputtered, evaporated or fabricated by other methodof thin film deposition. Other metals or thin multi-layer metal coatingsmay be also applicable. Typically, conductive layers in a flat-paneldisplay substrate are fabricated by sputtering which has limitations interms of step coverage. On the other hand, the roughness of theconductive layers such as metal films and ITO, increases with layerthickness. A thinner electrode layer produces a smoother surfacesuitable for OLED fabrication. This also reduces the cost of production.Therefore, a reduction of the thickness of the pixel electrode whilemaintaining its continuity over substrate profile is desirable.

If the through-via had a nearly vertical sidewall, the thickness of themetal to cover sidewall continuously, could be of the same order as thedepth of the via, which is equal to the thickness of the planarizationdielectric layer (in a range of few micron). In the pixel 10 of FIGS. 2to 3, the sidewall is made sloped rather than vertical. That permits thethickness of the pixel electrode to be reduced substantially in avertically stacked pixel structure.

FIG. 5 shows one example of a sidewall slop inside the via 8 of FIGS. 2and 3. In FIG. 5, an angle .beta. between the OLED bottom electrode 20on the sidewall and a TFT final material 54 is less than 90 degrees. Ifplanarizing polymer dielectric is formed from photosensitive initialformulation, the sloped sidewall can be achieved by means of appropriateexposure conditions.

An example of sidewall slope control in the through-via for BCB-layer(i.e., dielectric 22), which is produced from photosensitive initialmaterial, is presented in Table 2.

TABLE 2 Sidewall angle in cured BCB layer which was produced fromphotosensitive BCB-material as a function of exposure time Exposuretime, sec Sidewall angle, deg 20 45 30 33 13 27

The formation of the layer and patterning of the vias were achieved bymeans of spin-coating photosensitive BCB material with subsequentsoft-bake, light-exposure, developing, developer solvent removal andcure. In Table 1, the sidewall angle .beta. between the planarizationlayer 22 and the TFT final material 54 is shown as a function ofphotosensitive BCB-exposure time.

After the exposure, the film underwent 30 seconds post-exposure bake at55. degree C. and was developed for around 3 minutes in the developersolvent followed by 60 second bake at 75. degree C. for developersolvent removal and then final cure.

The conditions for spin-coating, soft-bake, exposure, post-exposure bakeand final cure are variable, and may depend on pixel designrequirements. Recommendations about process conditions of Photo-BCB aregiven, for example, by “Cyclotene™ 4000 Series Advanced ElectronicResins (Photo-BCB)” of Dow Chemical™, at hftp://www. dow.com/cyclotene/prods/402235 htm.

As shown in Table 2, the sidewall angle .beta. relates to the exposuretime. The sidewall angle .beta. becomes smaller when exposure time islonger. For example, for the sidewall angle .beta. of less than 45degrees and the planarization dielectric thickness of around 3 .mu.m,the continuous coverage of the via sidewalls was achieved with a pixelelectrode thickness of order 100 nm. This is much less than thethrough-via depth and enables the electrode surface of the OLED bottomelectrode 20 to be sufficiently smooth.

For polymer dielectric material made of non-photosensitive initialformulation, the sloped sidewall can be also achieved. For example, thiscan be done, by optimizing masking and plasma etching steps.

The parameters, materials and/or process of fabricating the slopedthrough-vias 8 are adjusted so as to: ensure the continuous sidewallcoverage by a material of the pixel electrode; make the roughness of theOLED electrode small enough (1 nm order) to prevent electrical shortsbetween the OLED top electrode 16 and the OLED bottom electrode 20.

One example of fabricating the pixel 10 of FIG. 2 is shown in FIGS. 6 to8. First, the TFT backplane 14 is fabricated (FIG. 6) on the substrate30. Next, the TFT backplane 14 is coated with a planarization layer 22,where the vias 8 with sloped sidewalls are opened to the selected nodesof the TFT backplane 14 (FIG. 7). For BCB planarization layer made ofphotosensitive formulation, BCB material is applied by spin coating, andprocessed including soft-bake, UV-exposure through a photomask,post-exposure cure, developing, solvent removal and final cure. Thissequence gives patterned material (with the through-vias 8) whose layerthickness and via sidewall slope depend on processing conditions, suchas the exposure time as described above. Typically, surface roughness ofcured BCB-layer is about 1 nm. Then, a thin residual layer on the bottomof the through-vias 8 is removed by plasma etching. Etching conditionsare optimized for short etching time and minimum roughening of the BCBsurface. For example, the fabrication of the pixel 10 may include plasmaetching in CF4+O2 gas mixture or SF6+O2 gas mixture, a combination ofhigh power high density plasma (for example, inductively coupled plasma)and low power reactive ion etching to achieve short etching time (few-20seconds); and virtually no change in roughness after plasma etching.

Subsequently, a conductive material is deposited and patterned to formthe OLED bottom electrode 20 (FIG. 8). Finally; the OLED layers 18 andtransparent electrode top electrode 16 of the OLED are continuouslyapplied over the pixels (FIG. 2).

The shield electrode 24 of FIG. 3 is now described in detail. As shownin FIG. 3, optional shield electrode can be incorporated in a pixelstructure. After formation of the TFT backplane 14, interlayerdielectric 21 is deposited. This can be done by means of CVD,plasma-enhanced CVD process or other method. Silicon nitride, siliconoxide or silicon oxide nitride with the thickness between 0.1 and 1.mu.m can be used as the interlayer dielectric 21. After formation ofthe vias in the interlayer dielectric that provide interconnectionbetween source-drain and shield metallization layers, shield metal layeris deposited and patterned to form the shield electrodes 24 andinterconnection plates 26. The interconnection plates 26 serve to carrythe potential from the certain node of TFT pixel, which can be eithersource or drain of a TFT, to the bottom electrode of OLED device 20.Then, the planarization layer 22 is applied and patterned, as describedabove, which is followed by deposition and patterning of OLED bottomelectrode 20, deposition of the OLED layers 18 and top transparentelectrode 16.

FIG. 9 shows another example of the pixel of FIG. 1. In FIG. 9, TFTsource/drain metal overlaps a contact plate 23 made of thin and smoothconductive material, such as Cr, Mo or other. The contact plate 23 isformed by deposition and patterning of conductive films on the flatportion of pixel area. Preferably, the thickness of the contact plate 23is between 50 and 150 nm.

The contact between the certain node of the TFT circuit, which is in thesource/drain metallization layer of the TFT backplane 14, and the OLEDbottom electrode 20 is made via the contact plate 23 rather thandirectly.

Depending on structure and fabrication method of the TFT backplane 14,the source-drain metal may have surface roughness well in excess of 1nm. This may be the case if relatively thick metal layer, especially Alor Al-alloy, is used for source/drain metallization. Such a source-drainmetallization can be required for the reasons associated with particularTFT fabrication process or display design. For example, highlyconductive routing metallization is beneficial for reduction of powerdissipation or better OLED brightness uniformity over the substratearea, especially if the display size is large. If such a source/drainmetal would be in a direct contact with the bottom electrode of the OLED20, its surface roughness is translated into the roughness of electrode20 inside via area. This can make this area a source of shortagesbetween OLED electrodes 20 and 16 and therefore cause OLED failure.Thus, in the pixel of FIG. 9, the contact to the bottom OLED electrode20 is made via the smooth contact plate 23 formed in separate layer. Inaddition, if the TFT final metal (source/drain-metal) 2 is Al orAl-alloy or the like, and the bottom OLED electrode 20 is conductiveoxide such as ITO, having a contact to the electrode 20 made of Cr, Moor the like instead of Al/Al-alloy will reduce contact resistance, heatdissipation in the contact and improve overall contact reliability.

The contact plate 23 is formed before source/drain metallization of theTFT backplane 14. The TFT source drain-metal, which is formed next, hasto overlap some portion of the contact plate 23 but leave a sufficientportion open for formation of via 8. In addition, it is desirable thatsource/drain metal can be selectively etched over contact plate metal.For example, if source/drain metal is Al or Al-alloy, using Cr forcontact plate would provide excellent wet-etch selectivity. Roughness ofthe order 1 nm is easy to achieve with thin layers of metals such as Cr,Mo, Ti produced by sputtering, evaporation or other methods. Appropriatethin multi-layer metal coating can off cause be also used for contactplate 23.

After the TFT backplane 14 with the contact plate 23 is formed, furthersteps, application and patterning of the planarization dielectric layer22, deposition and patterning of the bottom OLED electrode 20,deposition of the OLED layers 18 and OLED top electrode are performed ina manner described above.

FIG. 10 shows another example of the pixel of FIG. 1. In FIG. 10, thepixel has the shield electrode 24 and the contact plate 23. As describedabove, the shield 24 is formed to keep electric potential on the top ofthe TFTs at certain desired level. As the TFT backplane 14 is formed,the interlayer dielectric 21 is deposited. Then the contact plate 23 isformed of a thin and smooth metal layer such as Cr, Mo or the like on aflat portion of the pixel area. Preferably, the thickness of the contactplate 23 is between 50 and 150 nm. The vias in the dielectric 21 arepatterned to provide interconnections between the source/drain andshield metallization levels where necessary. Then, shield metal isdeposited and patterned to form the shield electrodes 24 and theinterconnection plates 26. The interconnection plate 26 is to overlapthe contact plate 23 but to leave its sufficient portion open, as shownschematically in FIG. 10. Preferably, the shield metal is selectivelyetched over the contact plate metal. As the TFT backplane 14 with theshield electrodes 24 and the contact plates 23 is formed, theplanarization dielectric 22 is applied and though-via 8 is formed on thetop of the portion of contact plate 23, which is free from shield metal(FIG. 10). Further steps (deposition and patterning of the bottom OLEDelectrode 20, deposition of the OLED layers 18 and OLED top electrode)may be performed in a similar manner as described above.

FIG. 11 shows a vertically integrated TFT-OLED pixel in accordance withanother embodiment of the present invention. The dielectric layer 22 andthe through-via profile of FIG. 11 are similar to those of FIG. 2.

The pixel 10 of FIG. 11 further includes an additional dielectric layer,i.e., dielectric cap 40, which is deposited on the top of the OLEDbottom electrode 20. The dielectric cap 40 is patterned so as to coverthe via area and the edges of the OLED bottom electrode pattern leavingthe rest of the OLED bottom electrode 20 uncovered. The OLED layers 18and the top electrode 16 are deposited in a similar manner as describedabove.

The dielectric cap 40 is provided to avoid breakage of continuouslydeposited OLED top electrode layers at the pixel edges, and therefore toprevent shortage of OLED devices. Further, the dielectric cap 40insulates the via area, which, depending on the structure andfabrication method of the TFT backplane 14, may have higher surfaceroughness than the rest of the OLED bottom electrode 20 and may betherefore a source of the shortage of the OLED device.

The dielectric cap 40 is made of material, which may be either polymerdielectric (such as, BCB, polyimide, other polymer dielectric) orinorganic insulator (such as, silicon oxide, silicon nitride, siliconoxide-nitride).

The thickness of the polymer insulator may be from a few 100 nm to a fewmicron. With the polymer insulator, as shown above, the sidewall profileof the cap pattern can be made smooth enough to enable continuouscoverage with the OLED layers 18 and OLED top electrode 16.

With inorganic insulator, the thickness of the dielectric cap 40 isadjusted in such a way to enable continuous coverage of the profilesteps associated with the cap layer by the OLED top electrode 16. Thethickness of an inorganic insulator can be between 50 and 500 nm (mostpreferably 50 to 200 nm). In addition, the conditions of dry or wetpatterning of an inorganic insulator, such as silicon oxide or the like,can be adjusted to form the sloped sidewalls.

One example of the fabricating process for the pixel 10 of FIG. 11 isseen from FIGS. 6 to 8, 11 and 12. As the TFT backplane 14 is formed onthe substrate 30 (FIG. 6), the planarization dielectric 22 is appliedwhere the vias 8 with sloped sidewalls are opened to the source-drainmetal 2 (FIG. 7). A conductive material is deposited and patterned toform the OLED bottom electrode 20 (FIG. 8). Then, the dielectric cap 40is disposed as described above (FIG. 12). Then, the OLED layers 18 andthe electrode are disposed and which completes the formation of thepixel structure shown in FIG. 11.

FIG. 13 shows a vertically integrated pixel in accordance with anotherembodiment of the present invention. The pixel 10 in FIG. 13 includesthe shield electrode 26 and the dielectric cap 40. First, the TFTbackplane 14 is fabricated followed by deposition and patterning of theinterlayer dielectric 21 and the shield electrode 24. The vias in theinterlayer dielectric are formed to provide interconnection between thesource/drain and interconnection plates 26 made in the shieldmetallization layer, where necessary. Next, shield metal is depositedand patterned to form the shield electrodes 24 and the interconnectionplates 26. Next, the planarization dielectric 22 and the OLED bottomelectrode 20 are deposited and patterned in a similar manner asdescribed above. Then, the cap dielectric layer 40 is disposed andpattern as described in the previous embodiment. Finally, the OLEDlayers 18 and the OLED top electrode 16 are formed.

According to the embodiments of the present invention, the verticalpixel integration provides higher aperture ratio, which leads to: thepossibility of using more advanced multi-transistor pixel driver circuitfor improved display performance without taking up extra light-emittingarea from the pixel; the possibility of using a TFT backplane, such asamorphous silicon, having lower mobility in contrast to poly-silicon,thereby simplifying the manufacturing process and reducing cost; and thereduction of current density through OLED providing higher operationalstability and improved lifetime of the display device.

Further, the fabrication process sequences and critical processingdetails described above solve a variety of issues pertinent to verticalintegration such as: smoothening out/planarizing vertical profiles inthe dielectric layer 22 of the structures on the TFT substrate 14 toenable continuity of the OLED device layers 12; continuous sidewallcoverage by pixel electrode material in the through-via profile in thedielectric 22; roughness of the order of 1 nm on the dielectric 22 andsubsequent electrode layer, which enables successful OLED fabricationand to higher yield rate; and capping structure feature which do notcomply to OLED fabrication process in terms of step height, sidewallangle and surface roughness by a dielectric layer. The via and edges ofthe electrode are covered with the dielectric cap 40.

While particular embodiments of the present invention have been shownand described, changes and modifications may be made to such embodimentswithout departing from the true scope of the invention which is definedin the claims.

1-20. (canceled)
 21. A pixel having a vertical architecture, comprising:an organic light emitting diode (OLED) device having a bottom electrode,one or more OLED layers and a transparent top electrode for emittinglight; a thin-film transistor (TFT) based backplane for electricallydriving the OLED device, the TFT based backplane being verticallyintegrated with the OLED layers and located below said bottom electrodeto form a top-emitting OLED, the TFT based backplane comprising: sourceand drain nodes; and a thin conductive contact plate formed entirely ona flat portion of the pixel area and electrically coupled to a source ordrain node; a planarization dielectric layer provided between the TFTbased backplane and the OLED bottom electrode so as to planarize thevertical profile on the TFT based backplane, said planarizationdielectric layer being in direct contact with both said TFT basedbackplane and said OLED bottom electrode; a via in said planarizationdielectric layer to provide a communication path between said TFT basedbackplane and said OLED device and through said thin conductive contactplate; and a dielectric layer deposited on top of said bottom electrodeand covering said via and all the edges of said bottom electrode whileleaving the rest of said bottom electrode uncovered.
 22. The pixel asclaimed in claim 21, wherein the sidewall of said via in theplanarization layer is sloped against the TFT based backplane.
 23. Thepixel as claimed in claim 21, in which said dielectric layer ispatterned in such a way that it insulates the OLED layers from the OLEDbottom electrode at pixel edges and in and around the via while leavingthe rest of the OLED bottom electrode in the direct contact with theOLED layers.
 24. The pixel as claimed in claim 21, wherein the pixel hasa roughness of the order of 1 nm on the planarization dielectric layerand subsequent electrode layer.
 25. The pixel as claimed in claim 21,further comprising continuous sidewall coverage by pixel electrodematerial in the via profile in the planarization dielectric layer. 26.The pixel as claimed in claim 21, further comprising a shield electrodeformed over the TFT.
 27. The pixel as claimed in claim 22, wherein theTFT based backplane includes: a substrate; an interlayer dielectriclayer on the source and drain nodes; and an interconnection platepatterned on a via of the interlayer dielectric layer and beingconnected to the source or drain node and the thin conductive contactplate; wherein the planarization dielectric layer planarizes thevertical profile on the substrate with the fabricated TFT basedbackplane, and the sloped via providing the communication path throughthe interconnection plate.
 28. The pixel as claimed in claim 27, whereinthe thin conductive contact plate is formed on a flat portion of theinterlayer dielectric layer such that the interconnection plate overlapsa part of the contact plate.
 29. The pixel as claimed in claim 27,further comprising a shield electrode disposed between the planarizationdielectric layer and the interlayer dielectric layer, which is locatedseparately from the interconnection plate.
 30. The pixel as claimed inclaim 22, wherein the TFT based backplane includes: a substrate; whereinthe thin conductive contact plate is formed such that the source ordrain material overlaps the thin conductive contact plate and theplanarization dielectric layer planarizes the vertical profile on thesubstrate with the fabricated TFT based backplane.
 31. The pixel asclaimed in claim 30, further comprising a shield electrode formedseparately from said thin conductive contact plate.
 32. The pixel asclaimed in claim 21, wherein the planarization dielectric layer includesphotosensitive benzocylobutene (BCB), the slope of the via beingadjusted by the exposure time of the photosensitive BCB.
 33. The pixelas claimed in claim 23, wherein said dielectric layer includes polymerdielectric or inorganic insulator.
 34. The pixel as claimed in claim 23,wherein said dielectric layer includes material selected from the groupfrom BCB, polyimide, polymer dielectric, silicon nitride and a thin filminorganic.
 35. A method of fabricating a pixel, the pixel having anorganic light emitting diode (OLED) bottom electrode, one or more OLEDlayers on the OLED bottom electrode, a transparent top electrode, and athin-film transistor (TFT) based backplane for electrically driving theOLED and including a substrate, the method comprising the steps of:providing the TFT based backplane on said substrate, including formingwithin the TFT based backplane a thin conductive contact plate entirelyon a flat portion of the pixel area and electrically coupled to a sourceor drain node of the TFT based backplane; providing a dielectric layeron the TFT based backplane, including the step of planarizing a verticalprofile in the dielectric layer so as to planarize the vertical profileon the substrate with the TFT based backplane, said planarizeddielectric layer being in direct contact with said TFT based backplane;forming a via in said planarization dielectric layer to provide acommunication path between said TFT based backplane and said OLED deviceand through said thin conductive contact plate; depositing said OLEDbottom electrode directly on top of said planarized dielectric layer andextending through said via into direct contact with said thin conductivecontact plate of said TFT based backplane; and depositing a dielectriclayer on top of said bottom electrode and covering said via and all theedges of said bottom electrode while leaving the rest of said bottomelectrode uncovered.
 36. A method of fabricating a pixel, the pixelhaving an organic light emitting diode (OLED) bottom electrode, one ormore OLED layers on the OLED bottom electrode, and a thin-filmtransistor (TFT) based backplane for electrically driving the OLED andincluding a substrate, the method comprising the steps of: providing theTFT based backplane on said substarte, including forming within the TFTbased backplane a thin conductive contact plate entirely on a flatportion of the pixel area and electrically coupled to a source or drainnode of the TFT based backplane; providing a dielectric layer on the TFTbased backplane, including the step of planarizing a vertical profile inthe dielectric layer so as to planarize the vertical profile on thesubstrate with the TFT based backplane, said planarized dielectric layerbeing in direct contact with said TFT based backplane; forming a via insaid planarization dielectric layer to provide a communication pathbetween said TFT based backplane and said OLED device and through saidthin conductive contact plate; depositing said OLED bottom electrodedirectly on top of said planarized dielectric layer and extendingthrough said via into direct contact with said thin conductive contactplate of said TFT based backplane, and depositing a dielectric layer ontop of said bottom electrode and covering said via and all the edges ofsaid bottom electrode while leaving the rest of said bottom electrodeuncovered.
 37. A method as claimed in claim 35, wherein forming said viawhich provides the communication path between the TFT backplane and theOLED device through the planarization dielectric layer, comprisesforming the via such that the sidewall of the via in the planarizationlayer is sloped against the TFT based backplane.
 38. A method as claimedin claim 35, wherein providing said dielectric layer between the OLEDbottom electrode and the OLED layers, comprises patterning thedielectric layer in such a way that it insulates the OLED layers fromthe OLED bottom electrode at pixel edges and in and around the via whileleaving the rest of the OLED bottom electrode in the direct contact withthe OLED layers.
 39. A method as claimed in claim 35, wherein theplanarization dielectric layer including photosensitive benzocylobutene(BCB), further comprising the step of adjusting the exposure time of thephotosensitive BCB such that the sidewall of the via in theplanarization layer is sloped against the TFT based backplane.
 40. Amethod as claimed in claim 35, wherein the pixel is formed such that thepixel has a roughness of the order of 1 nm on the planarizationdielectric layer and subsequent electrode layer.
 41. A method as claimedin claim 35, further comprising the step of providing continuoussidewall coverage by pixel electrode material in the via profile in theplanarization dielectric layer.